Angle of attack indicator



y 1944- Y c. L. JOHNSON 2,352,955

ANGLE or" ATTACK INDICATOR Filed Sept. 13, 1940 4 Sheets-Sheet 1 l6 l5 5IO E E 3 II 2 2 I Q i x 6 8 I8 21 9 57 x 5 l2 l3 F I ANGLE OF PITCHINVENTOR CLARENCE L. JOHNSON all ATTORNEY July 4, 1944. c. L. JOHNSONANGLE ATTACK INDICATOR Filed Sept. 13, 1940 4 Sheets-Sheet 2 INVENTOR QCLARENCE JOHNSON BY 5 'C a I 1 E w ATTORNEY y 4, 1944' c. L. JOHNSON2,352,955

ANGLE OF ATTACK INDICATOR I Filed Sept. 13, 1940 4 Sheets-Sheet 3 FIG-.3

INVENTOR CLARENCE L. JO H N SON ATTORNEY y 4, 4 c. L. JOHNSON 2,352,955

ANGLE OF ATTACK INDICATOR Filed Sept. 15, 1940 4 Sheets-Sheet 4 INVENTOR9 CLARENCE L JOHNSON BY M M AM ATTORNEY "leading to the tra airstreamwith respect to an airfoil and also a device ,most suitable angle ofattack ,stantial angle relatively to Patented July 4,1944

y UNITED v STATES PATENT I OFFICE I V ANGLE'OF mnroa'roa I 7 ApplicationSeptember 13,

14 Claims. (01. 73-180) This invention relates to improvements inindicating or warning devices for aircraft.

More particularly, it is an object of this invention to provide a devicefor indicating conditions of flight under which an airplane will stall.

It has been observed that conditions under which an airfoil will fail toreact on, or to be supported by, surrounding air do not occur at aparticular rate of flow of, the surrounding air relatively to theairfoil, but may occur within a considerable range of speed of theairstream.

Certain types of aircraft, for example, will stall at an air-speed of 60M. P. H. at one time and at 110 M. P. H. at another time.

For this reason an observation of an air-speed indicator will not 'givereliable indication of an approach to a flight condition under which theplane will stall.

'It is, accordingly, an object of this invention to provide an apparatusfor accurately ascertaining an approach to, or the existence of, aflight condition under which a plane will stall irrespec-' tive of theexisting air-speed.

I The invention is based on the observation that an airfoil will fail tobe, supported by the surrounding air when the airstream strikes theairfoil at more than a predetermined angle with respect to its axis. Forthe purpose of this description', the axis of an airfoil is understoodas being the longitudinal axis extendingfrom the edge in a verticalcrossthe airfoil. The angle of the to the axis of the airfoil angle ofatplane of with respect will hereinafter be referred to as tack." I

More generally it is an object of this invention. to provide anapparatus for dete direction of flow of a pressure fluid, such as air,for practicing this method. This device is particularly useful in:aeronautics'for determining the during flight, more particularly duringclimbing and landing.

It is, therefore, an object of this invention to sectional provideanapparatus'for determining the angle of attack or an approach toconditions under which an airplane will stall.

Broadly the mode of ms consists of the steps of deriving a first impulseproportional to the'square of the rate of flow of the surrounding airrelatively to the airfoil; deriving a second impulse proportional tooperation of my apparathe dynamic impact pressure exerted .at a sub- Iaxis of the airfoil,

exposed to the surrounding air impact pressures from 1940, Serial N0.356,620"

More specifically this invention aims at providing a simple apparatusfor. determining the angle of attack or conditions of stalling byutilizing two pressure impulses derivable from the surrounding air intwo directions or axes including a substantial vertical angle betweenthemselves.

Another object of the invention is to provide an angle ofattackindicator in which the parts are immovable.

The invention further aims at providing an indicating device in whichthe parts exposed to the surrounding air offer a minimum of resistanceto the air and are easy to keep free from ice even under unfavorableweather conditions.

More particularly, the invention aims at providing an angle of attack orstall indicator including a pressure head for deriving dynamic thesurrounding air which may be maintained free from ice by electricheating means in the same manner as electrically heated Pitot statictubes.

' The invention further aims at providing a pressure head for derivingtwo impact pressures from the surrounding air, the pressures being takenin two directions including a substantial angle betweenthemselves. Inthis specification the term impact pressure is broadly used as defininga pressure set up at a pressure port by flowing air directed at or pastthe same. This pressure may be positive if the flow of air is directedinto the pressure port or may be negative if the direction of the air issuch as to create a suction in" the port.

It is a further object of this invention to pro- Vide a pressure headfor deriving a static and two dynamic pressures from the surroundingair, one of which is proportional to the square of the air-speed.

Further aims objects and advantages of this invention will appear from aconsideration of the description which follows withaccompanying drawingsshowing for purely illustrative purposes embodiments of this invention.It is to be understood, however, that the description is not to be takenin aiimiting sense, the scope of this invention beingfdeflned in theappended claims.

ratio affording a meas-' trailing edge.

I In the drawings:

Fig. 1 is a view of an airplane illustrating the principle of theinvention;

from the air surrounding the plane. A first 1mpulse is taken which isproportional to the square of the rate of flight or air-speed. Thisimpulse may be taken from an air-speed indicator or most convenientlyderived as the dynamic pressure taken at the dynamic pressure port l3 ofa. Pitot static tube I4, this pressure being taken in a sub- Fig. 5 is afront view of the device shown in Fig. 4;

Fig. 6 is a side elevation partly in section of the device shown in Fig.4, a section being taken on line 6-6 of Fig. 4; 4

Fig. 'I is a plane view of a modified form of pressure head fordeterminin the angle of attack; 4

Fig. 8 is a front view of the device shown in Fig. 7;

Fig. 9 is a sectional elevation of the device shown in Fig. 7, thissection being taken on line 9-9 of section I; and

Fig. 10 is a diagramshowing a typical curve of angle of attack forvarious setting angles of the pressure head illustrated in Figs. 4 to 6.

The principle of the invention is illustrated in Fig. 1 showing anairplane having a fuselage Ill, The longitudinal substantiallyhorizontal axis of the fuselage, commonly called thrust line, isdesignated 2-2. The wing H of the airplane has a longitudinal axis 3-3extending from the leading edge or curved front to the This axis issubstantially horizontal but may include a certain angle 5 with respectto the thrust line 2-2 of the fuselage.

' When the plane is in flight the surrounding air, indicated by arrowsl2, will strike the fuselage and the wing at a certain angle. This angleof attack may either be measured as angle a with stantially horizontaldirection substantially parallel with the thrust line of the plane. InFig. 1,

the axis of the dynamic pressure nated 5-5. v Y

A'second impulse is derived which is proportional to the dynamic orimpact pressure exerted normal to an elementary area I 5' of theairfoil, thearea being inclined by a substantial angle with respect tothe axis of the airfoil.

In Fig. 1 the elementary ara I5 is shown inport I3 is desigclined withrespect to the thrust line 2-2 and the axis 3-3 of the airfoil II byangles a and a respectively. The direction or axis at which the pressureis taken is normal to the area and indicated by an arrow l6. This axisl5 forms angles a and a with the thrust line or the axis 3-3 of theairfoil II respectively. This pressure impulse must not necessarily betaken at the wing ll of the plane but maybe taken at an auxiliaryairfoil ll'having a dynamic pressure port l8, the axis of which includesa substantial vertical angle with respect -to the horizontal or the'axisof the airfoil proper.- In the illustrated embodiment the axis isdenoted I 5.

Best results are obtained if the angles 2 and c' are of the order ofless than 65. The axis along respect to the thrust line 2-2 or, moregenerally, as angle a with respect t an axis of an airfoil such as theaxis 3-3 of t wing II.

It will be noted that in the illustrated example the angles a and adifler'in magnitude by the amount A'the angle between the thrust lineand the axis of the wing ll. For'the sakeof clearness all angles in Fig.1 are exaggerated.

While it was previously assumed that the rate of flight or-air-speed ofan airplane aflords a measure of the condition under which the planewill stall, it was found that the same type of plane may stall at ahigher speed at one time and at a lower speed at another time dependingon various factors such as load, for example. It was found, however,that the same type of plane will stall when the angle of attacks: or aexceeds a a predetermined magnitude which is characteristic Y ratherthan pressure at its lower surface and that the suction at its uppersurface is created by air striking the leading edge of the airfoil at acertain angle. If this angle exceeds a critical value,

' the smooth flow of air at the upper surface creating the supportingsuction pressure will be disturbed and thesupport on the wing be lost.

Accordingto this invention the angle of attack maybe determined-byderiving two impulses which the pressure is taken, the direction I5normal to the elementary area forms, accordingly,

angles 6 and 6 of theorder of more than-25. I have found that optimumresults are obtained if the inclination of the elementary area and,accordingly, the direction of the axis ii at which the pressure istaken; forms a vertical angle of about 45 with respect to the axis oftheairfoil.

Of the two impulses thus obtained a ratio is formed, the ratio bemg ameasure of the angle of attack. Utilizing dynamic pressures, forexample, as hereinbefore set forth, the ratio p/q is proportional to theangle of attack wherein q is the impact pressure taken at the dynamicport l3 of the Pitot static tube and p is the impact .in the device ofFig. 2, the axis of the. airfoil 5'--5' is shown parallel to the axis5-5 of the Pitot static tube. As will be explained hereinafter ingreater detail, it is sometimes advantageous to tilt the axis of theauxiliary airfoil by an angle of pitch 7 with respect to the axis 5-5 orthe thrust line 2-2 in order to obtain a greatest possible change in theratio p/q for small changes in the angleof attack.

A first dynamic pressure duct l9 connects the dynamic pressure port 13of the Pitot static tube It with the mechanism of an indicatinginstrument 20. ,The pressure set up by the-air impinge ing upon the portIlls q and proportional to the square of the rate .of air-speed. Asupport 21 in the form of an inverted. Y to which the Pitot tube It ismounted supports the pressure head i1 having two pressure ports l8 and I'I'- forming an angle 8 with the axis 5-5. The pressure ports and actingphragm.

' strained for of the bell crank lever remain constant.

found that it is are obtained if the size of the pressure ports itone-sixteenth, preferably an inch. The indicator dial 23 graduated inangle and I8 is less than one-thirty-second, of is shown as having a ofattack.

A pointer by means of mechanism later to be described. Since the angle24 is movable relatively to the dial of attack is proportional to theratio p/q' the dial may also be graduatedto indicate ratio instead ofangle.

In Fig. 3 there is shown a form which the mechanism of the indicatinginstrument may assume.

Briefly the device operates by movement which is proportional to thelogarithm of the first impulse, pressure p, and by subtracting from thatfirst movement a second movement which is proportional to the logarithmof the second impulse, the pressure q. i

The difference between the two movements is then indicated on alogarithmically graduated dial.

In the illustrated embodiment a first dia-, phragm 25 connected to thepressure duct 22 is actedvupon by the pressure p. The diaphragm isrestrained to expand and contract proportional to the logarithm of thepressure p acting upon the same. This may be accomplished by means of aleaf spring 28 having adjustable stops 21 upon the centerpiece 28 of thediacreating a first The center piece carries a bracket 29 actuating abell crank lever 30 pivoted at 3 I. The bell crank 30 carries at itsfree end a pivotally mounted strip 32. This strip is engaged by andactuates a differential lever 33 pivoted at 34 and connected to atoothed segment 35 engaging a pinion 36 a pressure responsive actuatingof a pointer shaft 31. The pointer shaft carries a pointer 24 movableover the dial 23 carrying a logarithmic graduation 38. i

The movable strip 32 is further engaged by a second bell crank lever 39pivoted at 40 and acted upon by a second diaphragm 4| connected to thepressure duct 19. The second diaphragm is relogarithmic response bymeans of a having adjustable stops 43 and resting against the centerpiece 44. The center piece has a bracket 45 engaging the other endPreferably one arm of the bell crank lever is made adjustable. This isaccomplished in the illustrated embodiment by "an adjusting screw 46engaging the movable strip 32.

The operation of the-device is as follows:

Assuming first that the pressure q remains constant while the pressure pincreases, the bell crank lever 30 is moved in counterclockwisedirection about its axis thereby moving the differential lever 33 to theleft. ment the end 46 of the second bell crank lever 39 remainsstationary thus acting. as a pivoting support for the strip 32, assumingpressure q to The difierential lever 33 moves the toothed segment 35 andimparts a clockwise motion to the pointer 23.

In actual operation, how-ever,

leaf spring 42 During this movepressure p is usually accompanied by'achange in I because it makes the i maintain the tube structure free thepressure q. An expansion of to move to the 4| causes the end 46 of lever39 right thereby reducing the displacement of the differential lever 33by an amount proportional to the logarithm of the pressure q, therebyforming the difference between two logarithms or, in other words, theratio p/q indicated on the logarithmic dial 23.

The illustrated device is designed for use in connection with thepressure head in which the p pressures are positive. head is used' whichis so shaped or in which the pressure ports are so located that withinthe range in which the. instrument is used the p pressures are in thenegative, or suction range, it may be necessary to mount the p diaphragm25 to act upon the bell crank lever 30 in the opposite direction.

The dial may conveniently be graduated in angle of attack units ratherthan ratio units. The spring 39 serves to maintain the lever mechanismin engagement.

In Figs. 4 to 6 a preferred form of pressure head is shown for use in anangle of attack indicator. The device comprises a body or shell 41provided with a mounting fin 48 near its trailing edge for mounting to astrut or other form of support. Two pressure ports 49 and 50 areprovided at the curved front forming a vertical angle of preferably 45with the longitudinal axis 5'-5 of the airfoil. The pressure portscommunicate with the pressure duct 22 connectible to an indicatinginstrument. I

The use of two pressur ports is advantageous device non-responsive toslight amount of yaw, that is inaccuracies in the direction in which itis headed.

A different form of pressure head is shown in Figs. '7 to 9 including ashell 41, provided with pressure ports 49 and 50'. The pressure portscommunicate with the pressure duct 22. A dynamic pressure port isprovided at the front end of the tube at 5| communicating with apressure duct IS. A heater 52 may be incorporated to from lo under iceforming atmospheric conditions. Electric heater connections are shown at53.

In the illustrated embodiment the auxiliary airfoil or shell 41 hasrespect to the horizontal axis 55 by anangle qr. A permanent angle ofpitch 7 may be provided for in the manufacture of the tube.

Referring now to Fig. 10, it is seen that the p/q curve has a varyingslope depending on the angle 1 of pitch 7 at which the auxiliary airfoilmay be set. Since it is desirable to have a greatest possible change inthe indication for smallest changes in the angle of attack, that-is thedirection in which the surrounding air strikes the airfoil, it isdesirable to operate a portion of the curve which is steepest. In theillustrated curve the steepest portion of the curve is at an angle .ofpitch of approximately 15. It is, therefore, advantageous to provide foran initial angle of pitch of 15 in installing this pressure head orauxiliary airfoil.

If the airfoil is mounted with its axis 5-5' horizontal, the response ofan indicator operated by the tube to changes in angle of attack may beless, in other words, theamount of travel of the pointer will be reducedfor the same change in angle of attack. The most suitable angle of pitch7 for each auxiliary airfoil maybe deter- Obviously the presentinvention is not restricted If, however, a pressurean axis 5-5' inclinedwith to the particular embodiments herein shown and described. Devices01' different forms may :be

used for deriving p and q impulses. Other forms of ratio indicators maybe employed. Finally the device may be used in other positions than theones illustrated in the drawings. Quite obviously the device when turnedon itsrside will respond,

tions to obtain the ratio of said forces as an.

indication of the angle of attack.

5. In an angle of attack indicator for aircraft and the like, air foilmeans for deriving two forces,

said air foil means having at its leading edge a port Whose axis issubstantially parallel to the to the angleof yaw of the aircraft.Likewise the device is useful quite generally for determining thedirection of flow of a pressure fluid, pressure fiuid being understoodas being a gas as well as a liquid.

Moreover, it is not indispensable that all the second ports whoseaxes-are disposed at a vertical angle with each other, first and secondseparate hollow diaphragm capsules surrounded by atmospheric pressure,conduits separately connectingsaid capsules to said first and secondports, members movable by said diaphragm capsules according to thelogarithms of the pressures exerted on the said diaphragni'capsulesthrough said conduits and an indicating device including a lever systemoperative in response to movements of said members to indicated theratio of' the pressures from said ports exerted on said diaphragmcapsules;

2. In an angle of attack indicator for aircraft and the like, air foilmeans having at a leading edge a first port whose axis is substantiallyparallel to the direction of flight, said air foil means having at itsupper surface near its leading edge a second port whose axis is inclinedto the axis of said'first port, first and second separate hollowdiaphragm capsules surrounded by atmospheric pressure, conduitsseparately connecting said capsules to said first and second ports,members movable by said diaphragm capsules according to the logarithmsof the pressures exerted on said diaphragm capsules through saidconduits and an'indicating device including alever system operative inresponse to movements of saidmembers to indicate the ratio of thepressures from said ports exerted from said ports on said diaphragmcapsules 3. In an angle of attack indicator for aircraft and the like,airfoil means having first and second ports whose axes are disposed at avertical angle, with each other, first and second separate hollowdiaphragm capsules separately connected to said first and second ports,said capsules being surrounded byatmospheric pressure, means comprisingadjustable members for causing displacements of said capsules accordingto the logarithms of'the pressures impressed respectively thereon, andan indicating deviceresponsive to the difference in said logarithmicdisplacements to indicate the angle of attack.

4. In an angle of attack indicator for aircraft and the like, separatefirst and second air foils 'for deriving two forces, said first air foilhaving at its leading edge a force responsive portion whose axis issubstantially parallel to the direction of flight, said second air foilhaving near its leading edge a force responsive portion whose axis is ata vertical angle to direction of flight,

and means comprising mechanism for dividing pressures obtained from saidfirst and second por- '76 tally, said'body having on its upper surfacenear and to indicate angle or attack according to said ratio.

' 6. In an angle of attack indicator for aircraft and the like, air foilmeans having ports with axes disposed at a vertical angle, separatepressure diaphragmdevices communicating with said ports, said diaphragmdevices being surrounded by atmospheric pressure, a. leaf .spring drivenby each diaphragm, stop devices causing said springs to be displacedaccording to the logarithm of the pressure applied to its particulardiaphragm, a differential lever, connecting means for moving one end ofsaid differential lever in accordance .with the displacement ofsaid'first spring, connecting .means for moving the other end of saiddifferential lever in accordance with the displacement of said secondspring, a driven member engaging said diii'erential lever between theends thereof, and a.v spring pressed pointer operated ,by said drivenmember.

I 7. In an angle of attack indicator for aircraft and the like, air foilmeans having firstand second ports whose axes are disposed at a'verticalangle with each othenmeasuring devices responsive to air pressuresapplie'dat said ports, an indicating device for indicating angle ofattack, and

and the like, air foil means having an axis substantially p rallel tothe direction of flight, said air foil means having at a leading edgethereof a first-port whose axis is substantially parallel to thedirection of flight, said air foil means having at its upper surfacenear its leading edge a second port whose axis is inclined vertically tothe y axis of said first port, a first diaphragm responsive to thepressure impressed upon, said first" port, a second diaphragm responsiveto the pressure impressed upon said second'port, first and seconddisplaceabl elements operated respectively by said diaphragms, anindicating device for indicating angle of attack, and means com-'prisinga lever system for combining the movements of said displaceableelements to operate 1 said indicating device in response to ratio of thepressure obtained from said second port to .the pressure obtained fromsaid first port.

9. In an angle of attack indicator for aircraft I and the like, an airfoil comprising a pressure head including an aerodynamically shaped bodyhaving a leading edge and a trailing edge with a longitudinal axisextending therebetween, said axis beingdisposed approximat ly in the dirction of flight, said body having a substantially elliptical transversecross section with its longer transverse axis disposed substantiallyhorizonthe numerical values of V axis located substantially parallel tov 2,352,955 its leading edge a port comprising two horizontally spacedpressure openings symmetrically located with respect to saidlongitudinal axis, the axes of said pressure openings having a verticalangle of between and 65 degrees with said longitudinal axis, a ductcommunicating with said ports, means including a second duct terminatingin a second port facing the having an axis substantially parallel to thedirection of fiight, means connected to said ducts to respond tothepressures in said ducts, and indidirection of flight and.

eating means responsive to the. ratio of pressure obtained from saidfirst duct to the pressure obtained from said second duct of attack ofthe surrounding air on said aircraft.

10. In an angle of attack indicator for aircraft and the like, anaerodynamically shaped body having a leading edge and a trailing edgewith a longitudinal axis extending therebetween,

axis being disposed approximately in the direction of flight, said bodyhaving a substantially elliptical transverse cross sectionwith itslonger transverse axis disposed substantially horizontally, said bodyhaving on its upper surface near its leading edge two openingssymmetrically located with respect to said longitudinal axis, the axesof said pressure openings havingv a vertical angle of between 25 and 65degrees with said longiudinai axis, said body also having a secondpressure port on said having an axis parallel to said longitudinal axis,a strut connected to the after portion of said body and connected to theaircraft, ducts connecting said ports extending through said body andthrough said strut to the aircraft, means connected to said ducts torespond to the pressures in responsive to said first duct to thepressure I second duct to indicate the angle of attack of surroundingair on said aircraft.

11. In an angle of attack indicator for aircraft and the like, separatefirst and second air foils forderiving two forces, said air foilsadapted to be the mounted adjacent to each other but spaced from theaircraft said first air foil comprising a pressure head including anaerodynamicall shaped body having a leading edge and a trailing edgewith a longitudinal axis extending therebetween, disposed approximatelyin the direction of flight. said body having a substantially ellipticaltransverse cross section with its longer transverse axis disposedsubstantially horizontally, said body having on its upper surface nearits leading edge a port comprising two horizontally spaced pressureopenings symmetrically located with respect to said longitudinal axis,the axes of said pressure openings having a vertical angle of between 25and 65 degrees with said longitudinal axis, a duct extending throughsaid body and communicating with said ports; said second air foilcomprising a body having a longitudinal the direction to indicate theangle horizontally spaced pressure of fiight, and having a leading edgand a trailing edge, a pressure port on said leading edge having an axisparallel to the longitudinal axis of the body, a second duct extendingthrough said second body and communicating with its port, strutsconnected to the after portions of ence of said bodies and connected tothe aircraft, said ducts extending through said struts to the airsaidsecond duct to indicate the angle of the surrounding air on saidaircraft.

12. In a device of the character described, a first hollow chamber, afirst port at which a pressure p is exerted, a conduit connecting saidfirst port and first hollow chamber, said ,first chamber having adiaphragm which expands in response to an increase of the 9 pressure andcontracts in response to a decrease of 'the p pressure, a second hollowchamber, a second Port at which a pressure q is exerted, second port andsaid second hollow chamber, said second hollow chamber having adiaphragm which expands in response -to an increase'of the q pressureand-contracts in response to a decrease of the q pressure, and meansincluding a lever system operated by the movements of said diaphragms inresponse to the p and q pressures to indicate the ratio p/q.

13. In a device of first hollow chamber connected said first chamberhaving a diaphragm which expands in response to an increase contracts inresponse to a decrease in pressure at said first port, a second hollowchamber connected to a second port, said second chamber having a;diaphragm which expands in response to an increase in pressure andcontracts in response to a decrease in pressur at said second port,means to restrain the expansion ofsaid diaphragms proportionally to thelogarithms of the pressures at said ports, means operative in rethemovements of said diaphragms caused by pressures at said ports tosubtract the logarithmic said diaphragms and an indicator operative toindicate the difference in said logarithmic movements.

14. In a device of the character described, a

first hollow chamber, a first port at which is chamber having a responseto an increase decrease of the q pressure, an adjustable movable memberoperativeto restrain the movement of said first diaphragm and to bedisplaced according to of the pressure adjustable member operative torestrain the movement of said second diaphragm and to be the logarithmof' the pressure q, and means including a lever system operative "inresponse to displacement of said members to indicate the diiferthelogarithmic displacements of said members.

CLARENCE L. JOHNSON.

a conduit connecting said the logarithm

